Send for the cliche police, this was the ultimate game of two halves. Arsenal laid out their title credentials in the first 45 minutes – actually make that the first 10 minutes – then one interval and one sending-off later exposed their own defensive brittleness and allowed Newcastle to claim a seemingly impossible point.

Arsenal went in at the interval four goals up and believing they could be champions, or at least Manchester United's closest challengers. They went in at the end with 10 men, two points dropped and a furious Arsène Wenger waiting to speak to the referee. "We were very unlucky with some decisions that went against us," the Arsenal manager said. "I could not argue with Abou Diaby's red card, but I believe Joey Barton should have been sent off for the original tackle. My opinion is not important, though. We panicked a little bit in the second half, we have dropped two points, but psychologically the damage is bigger. We have a very disappointed dressing room."

No one could possibly have imagined such an outcome at the start of the game, when Arsenal looked deadly and Newcastle looked dead and buried. Striking even more quickly than Wayne Rooney had against Villa, Theo Walcott put Arsenal ahead after a mere 42 seconds, running into space behind the Newcastle backline to pick up Andrey Arshavin's flick from the centre circle and easily holding off Fabricio Coloccini to score. Anyone who imagined Newcastle could not have made a worst start was swiftly proved wrong. When Arshavin sent over a free-kick from the left, Johan Djourou was permitted a free header from six yards out to put Arsenal two up after three minutes. "You might as well go home," chorused the travelling supporters in the Leazes End.

It appeared the Newcastle defence already had. Pre-match speculation centred on how much the home attack would fare without Andy Carroll and Shola Ameobi – on his way to his radio commentary position Malcolm MacDonald lost count of the number of times he was asked whether he had brought his boots – yet the upshot of recent events seemed to be a complete loss of confidence throughout the team and an almost scandalous lack of concentration at the back.

Walcott had the pace and penetration to exploit the situation and he set up Arsenal's third after 10 minutes, skipping round José Enrique with embarrassing ease and crossing for Robin van Persie to stab a shot past Steve Harper. When the same player made it four inside half an hour, heading home Bacary Sagna's cross after neat interpassing on the right, Alan Pardew began to come in for sustained abuse from supporters immediately behind the dugout. The comments were audible because it was just about the only noise the home fans were making.

Cesc Fábregas and Van Persie brought saves from Harper before the interval, before the game turned on a meaty challenge by Barton a couple of minutes after the restart. Though Diaby was the injured party, Phil Dowd had little option but to take a dim view of the Arsenal player grabbing hold of his opponent by the scruff of the neck afterwards and flinging him to the floor. "The referee's decision was fair," Pardew said. "It was an aggressive tackle, that's all. I told the players at half-time I wanted aggression. We needed to show we cared. They went out and played like lions."

Newcastle got back into the game thanks to their remarkable supporters, as well as a penalty conceded by Laurent Koscielny and converted by Barton. As soon as the first goal of the fightback went in the grumbling ceased and the home side were roared forward. Arsenal crumbled with a suddenness that did not augur well for their title aspirations.

By the time Leon Best scored Newcastle's second he had already seen a goal chalked off for a borderline offside and, with Arsenal unable to subdue either Newcastle or their fans, more mistakes and more goals became inevitable. If Wenger could not see what the second penalty was awarded for he was not the only one. Neither Koscielny nor Tomas Rosicky appeared to do anything wrong, but Barton gratefully accepted his second invitation to score from the spot. The stadium was rocking by now and when Cheik Tioté brought the scores level, with a sumptuous volley after a Barton free-kick was only half-cleared, it duly exploded.

"Our fans will remember this for a long time," Pardew said. "When we went four goals down I thought the house might come down, but in the end we sent 51,000 Geordies home relatively happy. We were even a bit unlucky, because I thought we scored five good goals." Newcastle rarely managed that with Carroll, so perhaps they are not relegation fodder after all. And perhaps Arsenal will not be champions.

THE FANS' PLAYER RATINGS AND VERDICT

TOM HARBORD, YorkshireMags.com It was an unbelievable game. In the first half Arsenal were fantastic and we were awful, but it all changed after. Strangely it was disappointing from a footballing point of view, but very good for us. Their passing and movement were lovely to watch. Then once they were 4-0 up I think they thought they had it won and they totally lost discipline. All they had to do was sit back, really. But then we went at them – it was the attitude of the whole team but Barton and Nolan got things going and they were all pushing forward and having a go. Tioté looked good and is probably our best player at the moment.

BEN LOVER, Observer reader That was absolutely devastating. Even if you're down to 10 men – down to five even – it's just inexcusable not to be able to protect a 4-0 lead. Our play in the first half reminded me of the invincible team, dangerous on every attack, with pace and creativity, looking like we could take on anyone and win. Then the second half – at least one penalty looked very harsh, but that doesn't matter. There's no excuse to lose all organisation and intelligence. It's not the first time – we were up 2-0 against Spurs earlier in the year then lost 3-2. It's hard to take and understand, and a real blow to any title hope.